1. Field of the Invention
This invention relates to a hydraulic control system for an automatic transmission, and more particularly to a hydraulic control system which can properly control the operating timing of friction coupling members upon gear-shifting.
2. Description of the Prior Art
The automatic transmission for a vehicle generally comprises a torque convertor and a transmission gear mechanism, and the transmission is automatically shifted to a desired gear speed by switching the power transmission path in the automatic transmission gear mechanism by a selective actuation of a plurality of friction coupling members such as clutches and brakes. Such an automatic transmission is provided with a hydraulic control circuit which controls feed and discharge of hydraulic pressure to and from the actuators for the friction coupling members. The hydraulic control circuit comprises a regulator valve which regulates the pressure of the hydraulic fluid discharged from an oil pump to a predetermined line pressure, a manual valve for manually switching the driving range, a plurality of shift valves which change the hydraulic passage to the actuators and selectively actuate a plurality of friction coupling members according to the operating condition of the vehicle, and other auxiliary valves. Recently the shift valves are driven by solenoids in order to control the gear-shifting more accurately according to the operating condition of the vehicle.
Such an automatic transmission is sometimes arranged so that the condition of application (applied or released) of a plurality of friction coupling members are simultaneously changed upon a particular gear-shifting. For example, when four-speed automatic transmission has a first friction coupling member which is applied when the transmission is in third or fourth and a second friction coupling member which is applied when the transmission is in second or fourth, the first friction coupling member is applied while at the same time the second friction coupling member is released when the transmission upshifts to third from second. There has been known a hydraulic actuator for actuating the first friction coupling member which is caused to apply the first friction coupling member when an applying hydraulic pressure is supplied thereto and is caused to release the same when a releasing hydraulic pressure is supplied thereto even while the applying hydraulic pressure is being supplied thereto. When such an actuator is used, the first friction coupling member is supplied with the applying hydraulic pressure while the second friction coupling member is supplied with the releasing hydraulic pressure when the transmission is to be shifted from second to third.
A one-way clutch which selectively breaks and connects the power transmission path is sometimes provided in such an automatic transmission in order to smoothly effect the gear-shifting. In this case, the one-way clutch idles and the engine brake effect cannot be obtained when the vehicle coasts. In order to avoid such a problem, a friction coupling member for obtaining the engine brake effect is sometimes provided in parallel to the one-way clutch so that it is applied when the transmission is in a gear speed where the engine brake effect is required. When it is assumed that the friction coupling member for obtaining the engine brake effect is applied when the transmission is in third in the aforesaid automatic transmission which has the first friction coupling member which is applied when the transmission is in third or fourth and the second friction coupling member which is applied when the transmission is in second or fourth, the applying hydraulic pressure is supplied to the first friction coupling member, the releasing hydraulic pressure is supplied to the second friction coupling member and at the same time, the friction coupling member for obtaining the engine brake effect is supplied with the applying hydraulic pressure.
When the conditions of the application of a plurality of friction coupling members are simultaneously changed in order to effect a particular gear-shifting, it is important that the timing of changing the condition of application of each friction coupling member is properly matched with the timing of changing the condition of application of the other friction coupling members. For example, when the first friction coupling member is prematurely applied with respect to the release of the second friction coupling member upon 2-3 upshifting, the transmission will be momentarily neutralized and the engine will race. On the other hand, when the application of the first friction coupling member is too late, the transmission mechanism will momentarily fall into a double locking state and the output torque will be reduced, which leads to a gear-shifting shock.
In order to avoid the problems described above, the hydraulic control circuits are arranged so that the hydraulic pressure is supplied to and discharged from the respective friction coupling members at a proper engine speeds. However, conventionally, the timings of supply and discharge of the hydraulic pressure to and from the respective friction coupling members, the conditions of application of which are to be changed upon a particular gear-shifting, have been separately adjusted by the use of orifices, accummulators or the like. Accordingly, the change of the conditions of application of the friction coupling members are sometimes mistimed and the racing of the engine or the gear-shifting shock can be caused.
In order to overcome the problems described above, there has been proposed, in Japanese Patent Publication No. 57(1982)-20504, a hydraulic control system for an atomatic transmission in which the actual state of the automatic transmission is detected and the hydraulic pressures for the friction coupling members are controlled according to the actual state of the automatic transmission. However, this system is disadvantageous in that the actual state of the transmission is detected through the directions of the torques acting on the members for holding the friction coupling members or a direction of the relative sliding motion of the friction coupling members, and accordingly, the system is complicated in structure.